Precision tool and workpiece positioning apparatus

ABSTRACT

Apparatus for positioning a workpiece and tool in a precise location relative to each other by positioning in a predetermined site the workpiece, and then positioning the tool in a precise predetermined position relative to the site in minimum total time. The apparatus comprises a closed loop position and velocity sensitive servo system connected to the workpiece, the servo system including positioning apparatus for positioning the workpiece in a predetermined site. A position indicator determines the actual position of the workpiece relative to compared, fixed reference and emits a signal output which is compsred, in an error generator, with the desired position of the workpiece relative to the reference. The difference signal, from the error generator, is applied to the positioner and is used to bring the workpiece into the site. The positioner is provided with velocity feedback which cooperates with the positioning signal from the error generator to drive the workpiece into the predetermined site. Thereafter the error between the actual position address and the desired position address, although very small, is fed to second apparatus for positioning the tool a very small amount to precisely position the tool relative to the workpiece. The purpose of this abstract is to enable the public and the Patent Office to determine rapidly the subject matter of the technical disclosure of the application. This abstract is neither intended to define the invention of the application nor is it intended to be limiting as to the scope thereof.

Hassan et al.

[ 1 PRECISION TOOL AND WORKPIECE POSITIONING APPARATUS [75] Inventors:Javathu K. Hassan, Hopewell Junction; Carl V. Rabstejnek, WappingersFalls, both of NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Sept. 10, 1973 [21] App]. No.: 395,890

[52] US. Cl. 318/593; 318/601; 318/594; 318/617 [51] Int. Cl...G05B11/18 [58] Field of Search 318/593, 617, 601, 603, 318/594, 592;219/121 EB [56] References Cited UNITED STATES PATENTS 2,674,708 4/1954Husted 318/617 3,142,018 7/1964 Eisengrein 318/593 3,323,030 5/1967lnaba ct a1. 318/593 3,449,754 6/1969 Stutz 318/593 X 3,651,303 3/1972Rehme. 219/121 EB 3,719,879 3/1973 Marcy 318/593 3,733,484 5/1973Bayard..... 318/601 Primary Examiner-B. Dobeck Attorney, Agent, orFirmWilliam .1. Dick [57] ABSTRACT Apparatus for positioning a workpieceand tool in a precise location relative to each other by positioning ina predetermined site the workpiece, and then positioning the tool in aprecise predetermined position relative to the site in minimum totaltime. The apparatus comprises a closed loop position and velocitysensitive servo system connected to the workpiece, the servo systemincluding positioning apparatus for positioning the workpiece in apredetermined site. A position indicator determines the actual positionof the workpiece relative to compared, fixed reference and emits asignal output which is compsred, in an error generator, with the desiredposition of the workpiece relative to the reference. The differencesignal, from the error generator, is applied to the positioner and isused to bring the workpiece into the site. The positioner is providedwith velocity feedback which coop erates with the positioning signalfrom the error generator to drive the workpiece into the predeterminedsite. Thereafter the error between the actual position address and thedesired position address, although very small, is fed to secondapparatus for positioning the tool a very small amount to preciselyposition the tool relative to the workpiece.

The purpose of this abstract is to enable the public and the PatentOffice to determine rapidly the subject matter of the technicaldisclosure of the application. This abstract is neither intended todefine the invention of the application nor is it intended to belimiting as to the scope thereof.

21 Claims, 18 Drawing Figures Met-"E X-SERVO 194 cmcun mun Wm CONTROLCIRCUIT l r Y-SERVO CIRCUIT REBO LSAE PATEN IEU SEP 1975 SHEET Y-SERVOCIRCUlT FIGJ FIG.3

PMEN] EUSEP 9 $975 SHEET 2 FIG. FIG. FIG. 2A 2B 20 FIG.2

s? 58 I I i DISPL'AY 'DISPLAY DESIRED ACTUAL 54A ADDRESS ADDRESS 29A)100 50 5 V I I m AUTOMATIC-MANUAL swncumc cm 35A\ RESET ENCODER (HIGHORDER DIGITH FIG. 2A

PAIENItU 9W5 3.904.945

SHEET 7 o EXOR 0 2 1 c2 2 of 330 2 7 C22 22 c 7 EX OR AND AND OR '2 NORC25 7 3 V 2 CF 7 AND OR F 23 AND cm cf 7 1 FIG 8 2 A 2 2 fig BINARYBINARY ADDER ADDER I 72 B ems c 0 H8 7 AND CARRY OUT QR J (00) AND IFIG. 9

PATENIEU 9595 3, 904,945

sREEI 10 l FIG.15 129A 1/ DIGITAL R INTERFACE 57A KEYBOARD 4 1 5a DATAPROCESS PROCESSING CONTROL 5;: COMPUTER COMPUTER [MANUAL ADDRESS &

CONTROL 5 ENTRY l PANEL ELECTRO- 90A STATIC DEFLECTION AMP(DRIVER) 151FINE 97A OPERATION COMPLETE GATE J FIG. 14

PRECISION TOOL AND WORKPIECE POSITIONING APPARATUS SUMMARY OF THEINVENTION AND STATE, OF THE PRIOR ART The present invention relates topositioning apparatus, and more particularly relates to a workpiece andtool positioning system in which the workpiece is brought into apredetermined site or small area location and thereafter. utilizing thedifference between the desired and actual position of the workpiece, thetool is positioned into a precise predetermined location relative to theworkpiecev Numerous examples of servo positioning systems exist in theprior art, the servo systems containing means for positioning a loadwith positional velocity. or both types of feedback. Both types offeedback and combinations thereof are advantageous, positional feedbackdescreasing the rate of positioning to inhibit hunting, while velocityfeedback serves to proportionately dampen servo response. An example ofsome of the prior art is the patent to Allen, U.S. Pat. No. 3,241,015;McKenney, US. Pat. No. 2,9l3,649; Dickerson, US. Pat. No. 3,377,544;Husted, US. Pat. No. 2,674,708; and Plummer, US. Pat. No. 3,660,744.Many of the references above cited utilize both position feedback andvelocity feedback and combinations of both for ultimately positioningthe workpiece. However, when speed of positioning is important, it isdifficult if not impossible to achieve precise location of the workpiecewithout a certain amount of hunting and time for stabilizing (ring out)of the system.

Also described in the art is the provision to position a tool relativeto a workpiece, such as described I C Pattern Exposure by ScanningElectron Beam Apparatus by S. Miyauchi, et al., SOLID STA TE TECHNOL-OGY, July I969, pp. 43-48.

In machine tools, for example, the precise location of the workpiecerelative to the tool which is to perform the operation on the workpieceis extremely important, and if the operation is a repetitive one, thetime for ring out of the system and to stop the hunting of the systemfor operation of the tool on the workpiece can be an extremely importantfactor in system design effecting throughput. The total time for preciseworkpiece to too] positioning is composed of: workpiece Movementacceleration, constant velocity, deacceleration (which is virtuallyeliminated with the apparatus of the present invention) position huntingringout of residual energy, and secondary tool positioning.

In the positioning of, for example, an electron beam sometimes referredto as an E-beam, relative to a semiconductor chip in a semiconductorwafer, to write the circuit pattern onto the topology of the chip, theinitial precise location of the E-beam relative to the chip isabsolutely necessary to achieve the desired result, for subsequentworking of the chip by the E-beam. The system described hereinafter hasfound an advantage in the Ebeam technology because of its speed andpreciseness of registration of the tool to the workpiece. Utilizing theelectron beam, as an example, and other examples are given in thespecification, the philosophy of the system and apparatus will be moreeasily understood. Considering a workpiece and tool that must, at leastat the outset, be precisely positioned one relative to the other, if theheavier mass of the two may be positioned within a band of tolerance, orwithin the general locale of the position one relative to the other, thelighter mass or easier to move element, for example, the tool, may bemoved the remaining distance to position the tool in a precise locationrelative to the workpiece. An electron beam, for example, has theinherent capability of compensating for certain errors in the X and Yposition of the workpiece, as long as the error is known. Therefore,starting with this design philosophy, it is not necessary to positionthe XY positioning of the workpiece, such as a semiconductor wafer,within extremely small tolerance bands around an ideal position. It isrequired, however, to resolve the true position of the workpiece withinvery small increments, for example 50 microinches or less. Thus thesystem employed is a position feedback of the tables location. Onceworkpiece location is known and is set within a certain predeterminedtolerance area or site, movement of the workpiece or the XY table whichis used to position the workpiece, may be stopped and the additionalcorrection necessary to precisely position the E-beam may be applied tothe electrostatic deflection plates of the E-beam to position the beamin a precise manner relative to the workpiece. As may be expected.considerable time may be gained because the electromechanical drive ofthe heavier XY stage does not have to "hunt for the ideal position. Forexample, when the drive motor connected to the stage (workpiece stage)has been deaccelerated to the approximate location or site specified, itmay be stopped. The position feedback error may then be applied in apredictable manner to electronic compensation to the electrostaticdeflection plates of the E-bcam.

The basic velocity and position servo loop, which has already heretoforebeen alluded to. is a standard loop. However, recognizing the value ofan incremental distance to be moved, the intended usage in the systemdescribed is somewhat different. In the system, the loop is tuned toobtain a uniform introduction of energy (acceleration and constantvelocity), and a uniform removal of energy (deacceleration and stop).The intent is to effectively utilize the motors torque to move andarrest the workpiece or XY table as quickly as possible with a minimumof structural deformation of the mechanical components. Structuraldeformations which remain after the table is stopped represents energythat must be dissipated as vibrations or ring out. During this unstableperiod no operation may be performed. Accordingly, it is necessary totune the system to achieve an optimum tradeoff between the speed ofpositioning and ring out in order to achieve the shortest possible timeuntil work may be performed on the workpiece by the tool.

In view of the above it is a principal object of this system to provideminimum overhead time to position a workpiece relative to a tool and atool relative to a workpiece (tool and workpiece relative to each other)providing maximum throughput of operations of tool on workpiece.

Another principal object of the present invention is to provide aprecise workpiece and tool positioning system in which one of theworkpiece or tool is roughly positioned or positioned within a site, andthen the other of the workpiece and tool is positioned accurately withinthis site and at an exact position of the one relative to the other.

Another object of the present invention is to provide a dual servo loopsystem such that when the servo is within a small distance from itsdesired address, or when the difference between the desired and actualposition crosses zero (changes sign J, or the velocity decelerates belowa threshold velocity, the servo drive signal is switched and latched tostop whereby the servo is prevented from going into a searching orposition hunting mode thereby allowing the use of an underdamped servosystem and eliminating or decreasing the amount of current going throughthe motor when it is in the stop position avoiding the necessity forcooling because of no heat being produced in the motor.

Still another object of the present invention is to provide means forgiving the true or actual workpiece or element position so as to drivethe servo with the difference signal, i.e., the signal indicative of thedifference between the desired position and actual position of theelement.

Still another object of the present invention is to provide a controlsystem for element positioning relative to a second element, in whichthe control may be accomplished if desired, by any number of automaticpositioning systems such as employing a digital computer.

Another object of the present invention is to provide an absoluteaddressing system of the precise location of a first element so that itmay he stepped to an adjacent section for further work and still beprecisely positioned relative to a second element such as a tool.

Other objects and a more complete understanding of the invention may behad by referring to the following specification and claims taken inconjunction with the accompanying drawings in which:

FIG. 1 is a simplified schematic block diagram of the apparatus of thepresent invention;

FIG. 2 is a composite drawing illustrating the ar rangemcnt of FIGS. 2A,2B and 2C which are block diagrams in more detail of the complete systememploying the apparatus of the present invention;

FIG. 3 is a fragmentary side elevational view ofa typical X-Y stagewhich may be used with apparatus of the present invention;

FIG. 4 is a fragmentary plan view of a typical tool holder which may beemployed with the apparatus of the present invention;

FIG. 5 is a schematic diagram of a portion of the apparatus illustratedin FIG. 2.

FIG. 6 is a schematic diagram of a typical comparing means which may beemployed in the apparatus of the present invention;

FIG. 7 is a schematic drawing of a portion of the comparing meansillustrated in FIG. 6;

FIG. 8 is a schematic drawing of another portion of the comparing meansillustrated in FIG. 6;

FIG. 9 is a schematic drawing of another portion of the comparing meansillustrated in FIG. 6;

FIG. I0 is a schematic drawing of a latch stop circuitry which may beemployed in the apparatus of the present invention;

FIG. I I is a fragmentary schematic diagram of a portion of theautomatic manual switching gate utilized in the apparatus of applicantsinvention;

FIG. I2 is a schematic diagram of the ring out detection means utilizedin conjunction with the apparatus of the present invention;

FIG. 13 is a schematic block diagram of the circuitry for controllingthe tool;

FIG. 14 is another fragmentary schematic view of a modified tool andcontrol circuit therefore for driving electrostatic deflection plates;and

FIG. 15 is a fragmentary schematic view illustrating in block diagramanother control system which may be employed with the apparatus of thepresent invention.

GENERAL DESCRIPTION Referring now to the drawings and specifically FIG.1 thereof, a simplified functional block diagram of apparatusconstructed in accordance with the present invention is illustratedtherein. As shown, a first element or tool I0 is located adjacent asecond element or workpiece W mounted on an XY stage I 1. Stage drivemeans including motors l2 and 13 are connected respectively for drivingthe X and Y portions of the X-Y stage 11, the motors being responsive toa desired position address from a control unit 100. As illustrated, thedesired address of the position of the workpiece W, and thus the stageII in the X direction is controlled by a signal output along line 101 tothe X servo circuitry 20, while the Y direction of the XY stage iscontrolled by an address along line I02. emanating from the control 100,to the Y servo circuitry 103. As shown. actual positional feedbackinformation is fed back to the X and Y servo circuitry through feedbackloops 20A and NBA respectively. In a like manner, velocity feedback,within the stage drive means, is fed back respectively through lines 12Aand 13A.

In accordance with the invention, the circuitry in both the X and Yservo circuitry 20 and 103 is set so that when the X-Y stage 11 and thusthe workpiece W is within a preset predetermined tolerance, or within asmall site, (that is the address difference between the desired positionand the actual position is within a very small tolerance) the motors 12and 13 are latched Thereafter, the address difference which stillexists, albeit very small, is fed to an X-tool control circuitry andY-tool control circuitry 104 to effect a minor correction to the toolI0, causing the tool 10 to be realigned relative to the workpiece.

The System Because the Y servo circuitry I03, associated motor and Ytool control circuitry [04 is identical to the X servo circuitry, motor,and X tool control circuitry, only one such circuitry will be discussedhereinafter. However, it should be recognized that in order to achieveat least two degrees of freedom of the workpiece relative to the tool,each of the systems must be duplicated.

XY Stage 11 A portion of the XY stage 11 is illustrated schematically inFIG. 3, the stage comprising upper and lower platforms 11A and 118respectively, the lower platform I IB being connected in a conventionalmanner to a Rohlix (trademark of Barry Wright Corporation) 14' whichtraverses in the direction of the arrow 14A due to the rotation of theshaft 12B of the motor 12. The stage HA is connected, in a like manner,to the motor 13, which is carried by the stage or platform 11B, andeffects the motion of the stage 11A relative to the stage 118 into andout of the paper as shown by the tail of the arrow 118'. It should berecognized that the X-Y stage II may be of any conventional form as longas it may be driven in some manner by the motors l2 and 13 respectively,and mechanically designed to provide sufficient servo response (ie,taking into account such factors as stiffness, backlash, lead, inertia,

resonance, etc.)

In the first embodiment of the tool 10, as shown in FIG. 4, very minutepositioning of this tool may be obtained by a step and repeatmicropositioning table 14 illustrated schematically in FIG. I, and morecompletely in plan in FIG, 4. In the illustrated embodiment, the table14 is shown housing a drill chuck or the like 18 with a drill 18Adepending therefrom, it being desired to locate the drill precisely withreference to the workpiece W mounted on the X-Y stage 1]. To this endthe micropositioning table 14 includes a frame 14A which may beconnected to conventional apparatus for moving the member bothvertically, so that the drill comes into contact with the workpiece W,and horizontally, either manually or under preprogrammed automaticpositioning. However, inasmuch as this does not form part of the presentinvention, but is merely an example of the type of tool and thepositioning thereof which may be utilized in accordance with theinvention, the micropositioning table 14 will only be described in minordetail, a more complete description being set forth in Volume l2, No. 11, April l970 pages 1958, [959 of the IBM Technical Disclosure Bulletin.In the illustration of FIG. 4, the chuck 18 is mounted in the table 2which is suspended from the frame 14A as by identical leaf springs I,mounted at right angles to each other on the corners of the table 2. Thesprings have a high spring rate along the Z axis and lesser but equalspring rates along the X and Y axis of the table 2. The spring ratesalong the X and Y axis imparting repeatable positioning capability forthe table. Thus the spring rates provide a deflection proportional tothe X and Y forces imparted by force actuators or drive means 4 and 5.

The X servo circuitry, which is used as the example hereinafter, servesto take the desired position of the workpiece W, compare it to itsactual position, and then drive the stage until the workpiece fallswithin a reasonable tolerance of where it is supposed to be, hereinafterreferred to as site, and thereafter provide any difference between theactual and desired position signal to the tool control circuitry 75 todrive the tool to effect a precise positioning of the tool relative tothe workpiece. To this end, and referring first to FIG. 2C, the stagedrive means 21 includes the motor 12, a tachometer 22 which may beintegral with the motor 12 and which applies the feedback signal 12A toa tachometer buffer electronics 22A, which in turn applies a feedbacksignal through lead 128 to a motor drive amplifier 23, and then througha booster amplifier 24, which closes its loop by providing an input 24Ato the stage drive motor 12. The stage drive means 21 may be referred toas a servo drive amplifier with tachometer damping feedback, such asystem being shown in US. Pat. No. 2,674,708 to H. L. Husted. The motordrive amplifier 23 may be a standard off-the-shelf Inland Motor, ModelNo. EMl80l and purchased directly from the Inland Motor Corporation ofRadford, Virginia. The booster amplifier 24 is also standard and may beutilized, in accordance with Inland Motors instructions for boosting thepower output of the EMl80l from 25 to, for example, 200 watts. The drivestage motor 12 may be a standard servo motor such as an Inland Motors,Model No. NT 2909A, the tachometer 22 which,

while being separate, may be mounted integrally with the motor andconnected directly to the drive shaft which leads to the stage 11. Atypical tachometer is the Inland Motor Model No. TG-l 318C.

The tachometer 22 feedback signal is applied to the tachometer buffer22A which is an operational amplifier adapted to filter out the noisegenerated by the tachometer (brush commutation in the tachometer) whichgives a velocity (relative to voltage) feedback signal through line 128to the motor drive amplifier 23 causing the motor drive amplifier toeither increase or decrease its signal to the booster amplifierdepending upon the positional information received, and discussedhereinafter, to the motor drive amplifier 23 from its first input at23A. The tachometer buffer 22A is a purchased part and may be, forexample, a Philbrick operational amplifier such as their part number1016.

In order to stop the positional signal on line 23A to the motor driveamplifier when certain conditions exist, and therefore to stop the motor12, a stop switch 25 may serve to open the positional signal informationgiven to the motor drive amplifier 23. To this end, the stop switch, ineffect, is an on-off switch, and may he a dual single pole single throwsolid state switch such as the Dickson DAS 2137-1, although a relay withappropriate contacts may be utilized in lieu thereof. The operation ofthe stop switch in conjunction with the stop logic, is describedhereinafter.

Coupled to the stop switch is a band reject filter 26, to which thepositional information may be applied as at input 26A, the positionsignal passing through the band reject filter and the stop switch 25 tothe motor drive amplifier 23. The band reject filter 26 is a notchfilter which attenuates the amplitude of the drive signal at theresonant frequency of the X-Y stage. For example, if the table resonatesat 50 cycles, the band reject filter is tuned or otherwise designed forthat particular frequency to minimize the amplitude at that frequency.The concept of a band reject filter for this purpose is not new in theart, for example see US. Pat. No. 3,660,744 issued on May 2, 1972 toPlummer.

Although the desired position input may be fed to the X servo circuitry20 in either analog or digital form, the preciseness of digital form ispreferred for such input to the circuitry. Accordingly, in order todrive the motor drive amplifier and thus the servo motor 12, means areprovided for converting the digital information to analog information ora voltage level corresponding to the difference between the desiredposition and actual position, the digital difference preferably beingexpressed in the binary coded decimal format for ease of humanreadability, although it should be recog nized that conventional digitalbinary numbers may be utilized to control the system. To this end andreferring to FIG. 28, a digital to analog converter 27 receives adifference signal in binary coded decimal form through inputs 27A, andprovides an analog output along line 278 to a function generator 28, inthe present instance a square root function generator. The functiongenerator 28 differentiates the difference positional information comingin from the digital to analog converter 27 from output 278 which is avoltage level indicative of the difference in distance that the systemmust move to arrive at a predetermined position relative to some fixedreference. Inasmuch as the function generator is receiving an inputvoltage that corresponds to the ad M. a... a,

dress differential between the desired position and the actual positionof the stage at any instance of time, since the first derivative of aposition with respect to time is velocity, and inasmuch as it isdesirable to maintain a uniform acceleration or deacceleration of thestage by way of the servo motor 12, the square root function is utilizedin the generator 19 so as to make velocity dependent upon distance asopposed to being dependent upon time. For an example of a positioningsystem utilizing a function generator for this purpose. see US. Pat. No.3,241,015 issued on Mar. 15, 1966 to Allen. The function generator maybe an off-the-shelf module such as the model 4095/ made by Burr- BrownCorporation. The binary coded decimal number to analog converter 27 maybe one of several purchasable converters such as the Cycon lnc.converter Mod. CY2735, made by Cycon Corporation of Sunnyvale,California.

The square root function generator performs acceptably in theimplemented embodiment because inherent time constants in the hardwaretend to attenuate the instantaneous changes in acceleration at thestart, middlc and end of the stepping increment. This potential impulsefunction has been treated extensively in cam design literature, and iscommonly called jerk or pulse to define the instantaneous rate of changeof acceleration. A suitable treatment of pulse is in CAMS Design,Dynamics, and Accuracy, Harold A. Rothbart, John Wiley and Sons, lnc.,New York, I956, Chapter 2. The treatment deals with the parabolicfunction which the square root function theoretically generates and themore desirable cycloidal function.

The output of the function generator is through a gain control amplifier29 which serves to limit the voltage to the band reject filter 26 andultimately the motor drive amplifier 23. The gain control 29 may be setat some selected level, as by limiting the gain of the amplifier byinput 29A from the control 100, as when the stage ll approaches itsfinal position or site. There are three separate conditions which canapply a stop signal to the stop switch 25 and cause the motor driveampli fier 23 to prevent further driving of the motor 12, these being: losition stop. that is when the table reaches the position for which itwas intended i.e., within the site; (2) a positive limit stop; and (3) anegative limit stop.

To this end and referring to FIG. 2C, a stop logic circuit 30 isillustrated as having four inputs, a first input 30A from a latch stopcircuit 35, hereinafter described, and which indicates that the stage 11has reached its desired position or site, a second input 30B whichindicates that the stage 11 has travelled beyond its limit in thepositive direction. and a negative limit input 30C which indicates thatthe stage has moved in the negative direction into its limit switch. Afourth input, 30D, is applied to the stop logic circuitry to reverse thepolarity to allow the stop logic to come off the limit switches. Theoutput of the stop logic 30 is applied through line 25A as an input tothe stop switch 25.

Thus the stop logic 30 acts as an interface between the various stopcommand sources and the stop switch 25, which when open, stops anysignal from being applied to the motor drive amplifier 23 and stops themotor 12. To this end, and referring now to P10. 5, three TTL modulesG1, G2 and G3 make up the gating network for controlling the on-offstate of the stop switch 25 and therefore the motor 12. G1 may be a type7404 hex inverter, that is it has six inputs and six outputs. (il drivesG2 which is a type 7400 quad, two input NAND gate. The outputs of the G2module drive the G3 module which may be a type 7420 dual, four inputNAND gate.

The outputs from module G1, the hex inverter, are always opposite theircorresponding inputs. The operation of the NAND logic blocks, G2 and G3can be explained by saying that the only way to get a logic low outputis to have all inputs at a logic high. All logic lows or any high, lowcombination on the NAND inputs results in a logic high out.

Referring now to the three ways in which the stop logic 30 effects astop or opens switch 25, the various stop signals 30A30C will be treatedserially.

l Position Stop When the table or stage 11 reaches near the position orsite intended, a logic high is applied to input 30A to the stop logic30. This input emanates from the latch stop circuit 35 which will bediscussed hereinafter. The logic high from input 30A is then applied toan inverter GlA which produces a low at one of the inputs of NAND gateG3. Although NAND gate G3 has three other inputs, regardless of theirlevels, a low at the first input to G3 (i.e., the output ofGlA) demandsthat the output of G3 on line 25A. is high. (Any low applied to theinput of a NAND gate produces a high output).

2. Positive limit stop If for any reason the stage arrives at the end ofits travel in the positive direction, a positive limit stop for examplea limit switch (not shown) will stop the motor. When the table is notagainst the positive limit stop, the input at 3013 is a logic high,which is provided by pullup resistor R1, typical values of which aregiven in FIG. 5. lfa positive limit is reached, a logic low is appliedto inverter G18. G18, in turn, places a high on one input of G2A.Inasmuch as the motor 12 was moving in a positive direction, the signinput at 30C 1 is at a logic high which is applied to the second inputof NAND gate GZA. The output, therefore, of NAND gate G2A goes low whicheffects a high or an up output along line 25A from NAND gate G3.

3. Negative limit stop Should the table reach a negative limit, that isstrike a limit switch (not shown) indicating that it had travelled toofar in the negative direction, the high level on the input to GlD isreplaced by a low, the high level being provided initially by pull-upresistor R2. The output of inverter GlD, which is high therefore, isapplied as an input to NAND gate GZB. Since the motor 12 was moving in anegative direction (in order to hit the negative stop) the sign inputalong input 30C1 is at a logic low. Because the l ow is applied toinverter GlC, the upper input to NAND gate G2B is a logic high causingthe NAND gate GZBs output or input to NAND gate G3 to be a low,effecting a logic high output on line 25A from NAND gate G3.

Simply stated the limit stops, above described, inhibit motion when alimit switch is actuated in the direction toward the limit switch.Alternately, the motion in the direction which would back the table offthe actuated switch is not inhibited.

Thus all three stopping modes effect a high output at G3. Such a highoutput along line 25A causes the stop switch 25 to open therebypreventing further driving of the motor 12.

In order to provide the digital to analog converter 27, via its inputbus 27A with a binary coded decimal difference between the desiredposition and actual position of the stage or workpiece, comparing means31 is provided. in the present instance the comparing means comprising adigital error generator. To this end, and referring first to FIG. 2B,the digital error generator includes a first or primary input 3IA towhich is fed the BCD address corresponding to the desired position ofthe workpiece or stage relative to some fixed reference. A second inputbus 318 feeds a binary coded decimal number corresponding, at any oneperiod of time, to the actual position of the workpiece or stage from aposition indicating means 40, which is described hereinafter. Althoughthe digital error generator may be of conventional structure, theparticular structure utilized is best illustrated in FIGS. 69.

The digital error generator takes the form of a binary coded decimalsubtractor. To subtract two numbers of the base 10, the subtrahend is 9scomplemented (each digit is subtracted from 9) and then added to theminuend. In this system. if the sum has a carry, the carry is broughtaround and added to the least significant digit (LDS) of the sum. Thistechnique is commonly referred to as end around carry." The sum is thenequal to the dividend of the original subtraction. If the sum does nothave a carry, then the sum is 9's complemented. This means that thedividend is negative. Thus the digital representation of the stageposition which is referred to in the drawings as input B, correspondingto the bus 318, is subtracted from the desired position. referred to inthe drawings as input A corresponding to the signal from bus 31A, usingthe 9s complement method. The dividend is converted by way of thedigital to analog converter 27 and, in the manner heretofore described,fed to the motor drive amplifier 23.

It should be noted that the subtraction could be made using a scomplement method (each number is subtracted from 9 and then 1 is addedto the result). This would not require the carry to be added to the sum,but does require additional logic necessary to add I when the number iscomplemented. As in the 9's complement method, if there is no carry thesum is then lOs complemented and considered negative. If there is acarry, it is just ignored. However, because of the additional circuitryrequired for the 10's complement method, the 9s complement method may bepreferable, and as illustrated in FIGS. 6-9.

Turning now to FIG. 6, the overall scheme of the comparing means ordigital error generator 31 is illustrated therein. In the embodimentshown, the input A corresponds to the address of the desired position ofthe table, while the input B corresponds to the address in binary codeddecimal numbers of the active position of the stage or table. Asillustrated, each pair of inputs is coupled to a BCD adder 32A. 328through 32N, each of the adders having an output therefrom coupled togated 9s complement circuitry 33A, 33B, 33N. The dividend or output fromeach of the gated 9s complement circuitry blocks is the binary codeddecimal number corresponding to the difference between the address ofthe actual position and the address of the desired position. For ease ofreading, the least significant digit (LSD) to the most significant digit(MSD) has been labelled both at the input and output. As isconventiona], each of the binary coded decimal (BCD) adders 32A-32N hasa carry input (CI) and a carry output (CO) each preceding carry out (CO)being coupled to the next succeeding carry in (CI) except the last carryout in the binary coded decimal adder 32N, corresponding to the mostsignificant digit MSD, is coupled to an inverter 34 the output of whichis coupled to each of the gated 9s complement 33A-33N and the input ofwhich is coupled to the carry in (CI) of the binary coded decimal adder32A. Additionally an output indicative of the sign, as marked, is takenfrom the carry out of adder 32 N (MSD). As illustrated each of the Binputs, before being applied as an input to the binary coded decimaladder passes a 9s complementing circuit designated 31B] 31B2, and 31BN.In operation, input A is the minuend while word or input B is thesubrahend, the subtrahend being complemented by the 9s complementcircuit and then added in the adder to word A. In the instance wherethere is no carry, the sum of A and B (A B) is complemented. On theother hand if there is a carry, the sum is increased by 1 but notcomplemented. This occurs in the gated 9s complemented circuitry.

Each of the 9's complement circuits includes an inverter 31C, andexclusive OR logic gate 31D and a NOR gate 31E, the inverter 31Creceiving the least significant bit of the group of bits 2 2". As shown.the next most significant bit, 2, is coupled directly to the output,while it is also provided as an input along with 2 bit to the exclusiveOR gate 31D to provide an output 2 The NOR gate 31E receives inputs fromboth the 2' and 2 as well as the 2 bit, the gates acting in aconventional manner to apply an output therefrom which acts as the inputto its associated binary coded decimal adder. To aid in an understandingof the design philosophy of the 9s complement and the gated 9scomplement circuits, the following table with notes is supplied.

No Compl. BCD (.OMPL.

U 9 (l (J (I (J l U (l l l 3 (l (l l l ll l) U 2 7 I) U l) (l l l l 3 (wl) l) l l U l I (l 4 5 l) l (l U l) l U l S 4 (l I ll I ll l (l (J (v 3U l I ll ll 0 l l 7 2 U l l I U l) l (l X l l ll (l (l U l) (l l 9 U l U(I l (I l) U ll 2" is always inverted in complementing 2' is neverinverted in complementing 2 is inverted only when 2' is high. The twoinversion states being (K) (I (ll l exclusive or Ill l l l O 2" isinverted when 2 and 2 are 0 all zeros l NOR any not zero l) the inverterand the AND gates is the number hecomes 9 complemented.

FIG. 8 illustrates the complemented digits by C2" and the uncomplementeddigits by G2". The exclusive OR 33D essentially performs the samefunction as the gut ing ANDs 33B and 33C and an inverter needed tocomplement the 2 bit, and is an equivalent circuit. Similarly the 2 bitreduces to a single line as there is never an inversion.

The binary coded decimal adders 32A32N may be constructed of standardparts as shown in FIG. 9. For example, a pair of conventional binaryadders may be joined as illustrated in the drawing. The inputs to thefirst binary added 36A are the binary coded decimal number correspondingto the desired position and the binary coded decimal numbercorresponding to the actual position which has been 9 complemented. Thepair of AND gates and OR gate are utilized in conjunction with theoutput of the first binary added to render a carry out or carry overposition to the next succeeding binary coded decimal adder.

The position indicating means signals the digital error generator 31 asto the position of the stage II, during any particular point in time,and includes an encoder position register 41 which sends a signal alongline 41A of the actual address of the stage in binary coded decimalnumbers. The output 4IA is branched into 41A], and 41A2, the branch 41A]providing the B input as described relative to FIGS. 6-9, to the digitalerror generator 31. The second branch, 4IA2 may or may not be utilizeddepending upon the control system employed. Various control systems willbe discussed hereinafter. The encoder position register 41 has a secondoutput preferably to a display of the actual address 42, which is aconventional display illustrating the position of the table in itsbinary coded decimal number form at any one period of time so that themachine operator, if the machine is operating under manual control, maydetermine the exact position of the stage. The encoder position register41 receives an input from an electronic reading head and light source 43which is optically coupled to a movable scale 44 mounted on the stage.the reading head and light source continuously monitoring the exactposition of the stage relative to a fixed reference.

The position indicator means may be purchased as an off-the-shelf item,and may be one of several types. The system employed, however. is theTeletrak an absolute digital readout system, supplied by WhitwellElectronic Developments Ltd. at 11 Watt Road, Hillington, Glasgow.

The basic requirement of numerical position measuring systems is theconversion of the analog to digital values because the information inthis particular format is better suited for data processing and providesa much higher resolution than may be obtained from practical analogdevices. Additionally, as with the use of the display of the actualaddress, the information may be easily converted to a numerical displayfor human read ability.

There are several types of analog to digital converters when absolute oractual position information is necessary, one of the principal types hasbeen the rotary encoder or digitizer. One version of the device (rotaryencoder) translates angular position into digital values by the use of amechanically rotated shaft with pickup brushes in contact with a statorin the form of a coded disk. The disk includes a plurality of concentricrings which are divided into equal conducting and nonconducting sectorsin either a binary or a binary coded decimal sequence. Because ofmechanical imperfections, and the small size of pickup brushes,positional errors arise from the use of the coded disk type encoderdigitizer. With the disk type encoder or digitizer, therefore, it isnecessary to take special precautions to avoid ambiguity in the readingfrom the positioner.

A preferred type of position indicating means is an optical defractiongrating. When two gratings are suitably arranged, interference patterns(MOIRE fringes) are produced ofa wavelength equivalent to the pitch ofthe gratings and of dimensions suitable for use in metrology. Thegratings may be manufactured in a conventional manner and themaintenance of a uniform gap between gratings and the relative parallelmovement are not excessive and are well within the limits of normalaccurate machine practice.

A more complete writeup of the Teletrak system may be found in theAugust 1969 issue of Automation (an English publication).

The digital error generator continuously provides a flow of informationto the digital to analog converter 27 and thus to the drive motor 12,the information being in the form ofa signal indicative ofthe differencebetween the actual position of the stage or table and the desiredposition. At some time when the difference between the desired positionand the actual position reaches a finite or SmaILdifference, it isnecessary to transmit a signal to the stop logic circuitry, and thus thestop switch 25, to open the flow of information to the motor driveamplifier 23 and thus stop the motor 12. To this end, the digital errorgenerator 31 also transmits a signal to a position stop generator 38from and along line 388. In its simplest form, the position stopgenerator 38 is a comparator, which takes an input comprising thedifference between the actual and desired table or stage position, andcompares it with a present predetermined value such that when thedifference signal is equal to or less than the preset difference, anoutput signal is generated along line 38A to the latch stop circuitry 35so as to transmit a signal along line 35B, and 30A to the stop logic 30(see FIG. 5) thereby opening the circuit and stopping all positionalinformation from being transmitted to the motor drive amplifier 23.

It should be recognized that other methods may be employed to set theposition for stopping the drive signal from being transmitted to themotor drive amplifier 23, for example, a signal input from the digitalerror generator indicating a sign changeover from, for example, plus tominus may be utilized in conjunction with the stop logic 30 to effect astopping of the transmitted signal to the motor drive amplifier.Additionally, a differential voltage indicative of velocity may be takenfrom the tachometer buffer 22A and directed to the position stopgenerator 38 such that when the voltage generated by the tachometerreaches a predetermined lower limit or changes direction, the comparatorreceives and compares it with a predetermined voltage causing thecomparator to change state and provide an output which effects a stopsignal through the latch stop circuit 35, via output line 38A, to thestop 30.

in order to provide a latching condition to the stop logic 30 undercertain predetermined conditions, a latch stop circuit is provided. Thepurpose of the latch stop circuit 35 is to generate a continuous stopsignal to the stop logic 30 when one of the stop conditions. such as adigital error less than some preset, predetermined value. generated bythe position stop generator 38, even after such stop condition may nolonger exist. Once such stop condition is latched it is then necessaryto re-establish a new go signal 35A (see FIG. 28) to again actuate orenergize the motor 12. This condition is necessary inasmuch as it ispossible that the stop condition which generates the stop signal maycease to exist even though the stop signal is still desired. For example, if the comparison signal in this position stop generators 38fixed input is very small it is possible to coast through the siteeffecting an opposite sign difference signal before stopping. Without alatch the stop signal would cease and the drive motor 12 would againdrive the tabe in the opposite direction precipitating a hunting modeposition servo which, as set forth in the objects, is an objective ofthis invention to eliminate. The latch is also designed to provide forstopping of the motor I2 prior to a generated stop signal by droppingthe go signal 35A which enables manual or automatic stopping due to someemergency or desire for a premature stop.

Referring now to FIG. 10, the latch circuit of the latch stop 35 isillustrated therein. A go signal on input 35A received from theautomatic/manual switching gate 50 and thus the control 100, serves toset the flipflop 35E which raises the level of the stop/no stop signal358. the signal 358 being utilized as an output to the input 30A of stoplogic 30 (see FIGS. 10, 2B and 2C). Go signal 35A, also serves to clearthe flip-flop 35F whose output 35D feeds back to NAND gate 35G and ANDgate 35H which control the flip-flop 35E. As shown, a reset output 35C,which is the opposite of the signal on 358, is fed back to theautomatic/manual switching gate 50 to indicate the latched and unlatchedcondition of the stop 35 and thus the motor 12.

As has already been explained when the stage has reached its approximateposition, within a predetermined site, the position stop generatortransmits a signal which sets the stop logic 30 and thus opens the stopswitch 25. Prior to starting the circuits which control the tool, it isnecessary to ensure that all motion of the stage has ceased. To thisend, a ring out detection circuit 45 is provided intermediate the toolcircuitry (each of the X and Y tool circuitry 75 and 103) and theencoder position register 41 (see FIG. 2B). The ring out detectioncircuit is best illustrated in FIG. 12 wherein an input signal isreceived from the encoder position register from line 45A, the inputsignal comprising, in the present instance, the least significant digit.The least significant digit input from line 45A is coupled to a firstand second row of inverters, the first row including inverters 46A, 46B,and the second row including inverters 47A, 47B, 47C, and 47D. In thefirst row, intermediate inverters 46A and 46B is a capacitor 46C whichis connected to ground, while intermediate inverters 47B and 47C in thesecond row is a second capacitor 47E connected between that row andground. Each of the rows terminates in a first input I to an AND gate 48and a NOR gate 49 respectively, the second inputs being cross-coupled sothat the output of inverter 47D comprises the first input of NOR gate 49and the second input of AND gate 48, while the output of inverter 468comprises the first input of AND gate 48 and the second input of NORgate 49. The outputs 48A m M... A... a.

and 49A are connected from their respective AND and NOR gates to an ORgate 50' which has an output therefrom 45B indicative of a stop ringingcondition. To aid in understanding of the operation of the circuit, theinput signal from line 45A is designated (M) and the output frominverter 46B is labelled (N), while the output from inverter 47D islabelled (P), therefore both (N) and (P) are applied to both the AND andNOR gates.

The circuit illustrated in FIG. 12 is used to detect when the last bitof the position encoder has stopped changing for a fixed period of time,for example the time A,. As may be seen, (N) follows (M) when (M) goesfrom a low to high, but has a certain time delay A, due to the action ofthe capacitor 46C, when (M) goes from high to low. Alternatively (P)follows (M) when (M) goes from high to low but has a delay time, A,,when (M) goes from low to high. As long as M changes state at a rateless than A,, then (N) cannot equal (P). However, when (M) changes stateat a rate greater than A,, then (N P). Accordingly, the output will behigh when the input remains in either state for longer than A,. Thus byaltering the size of capacitors 46C and 47E, for example, the ring outtime may be adjusted so that when the least significant digit (LSD) fromthe encoder position register 41 changes at a predetermined slow ratedue to the dumping characteristics of the stage, the output signal alongoutput line 45B of the ring out detection circuit 45 will go highindicating a stop ringing condition.

The automatic manual switching gate 50 is nothing more than a pluralityof switches to receive an input from either a manual control orautomatic control circuitry 100, discussed hereinafter. To this end, andturning now to FIG. 11, wherein a portion of an auto/- manual switchinggate circuit is illustrated, the circuit 50 includes a signal input 50Awhich emanates either from the automatic portion of the control circuitI00 or from the manual control circuit, the signal being utilized toclose either the automatic NAND bus circuit 51, or the manual NAND buscircuit 52. Each of the blocks 51 and 52 illustrated in the drawing maybe comprised of, for example, a flip-flop or relay having an outputwhich sets a plurality of switches to permit a flow of information fromeither the automatic input (Al-A4) or the manual input (Ml-M4) tovarious parts, heretofore described, of the system If each of thecircuits S1 and 52 require an up level input, (digitized 1) an up inputto the NAND bus SI will affect switch closure and permit the inputs A]through A4 to be provided to the NAND gates 53A, 53B, 53C, 53N, while alogic high input, when applied to the inverter 54 will cause an invertof the logic high giving a logic low keeping the buss 52 open so thatmanual input of the system is not possible. Alternatively, a logic lowapplied to the inverter 54 will cause such an inversion in inverter 54closing the switches in NAND bus circuit 52 and allowing the manualinput of signals (Ml-M4) to the NAND gates 53A-53N. Thus the NAND gates53A-53N may receive an input from either the automatic section of thecontrol circuitry or the manual section depending upon the activation ofany particular bus 51 or 52.

As shown in FIG. 11, various of the outputs of the NAND gates arelabelled, for example NAND gate 53A provides an input to gain control 29to allow presetting of the gain controls gain so as to reduce thevelocity of the motor 12 when desired. Additionally, the NAND gate 538which receives an input A3 and M3, serves to transmit an output signal54 which branches into an output 54A to a gate (described hereinafter)and to the line 35A to effect an unlatching of the latch stop 35 (seeFIG. 2A and 2B). The NAND gate 53C receives an input from either thesignal M2 or A2 depending upon which NAND bus circuit 51 or 52 isenergized, the signals A2 and M2 serving to reset the encoder positionregister 41, as by a signal 55 which inputs the encoder positionregister 41 with the high order digits and bits to reset the encoder.Inputs Ml and Al feed a plurality of NAND gates 53N through one of theirrespective NAND bus circuits 51 and 52 when energized, providing anoutput along bus 31A of the desired stage position relative to somefixed reference.

There are, of course, inputs as well as other outputs that may be usedin accordance with the embodiment shown in FIG. 11, for example an inputregister may include a plurality of NAND gates such as 53A-53N toreceive various input information, such as an output 45B! of the ringout detection circuit 45 indicating that the stage has stopped; theoutput of the encoder position register 41 along line 4lA2 indicatingthe actual position of the stage; and input from latch stop 35 alongline 35C (see FIG. 10) to indicate to the control circuit 100 thecondition of the latch stop and thus the stop logic 30. The output ofthe NAND gates receiving such inputs would be split into NAND buscircuits similar to those described heretofore relative to circuits 51and 52, so that when energized, the bus would provide an output to therespective one of the controls, either automatic or manual that had beenenergized. Other outputs are taken from the automatic manual switchinggate to, for example, displays 57 and 58. For example, display 57 may bea binary-coded-decimal number display of the desired address while thedisplay 58 illustrates the binary-coded-decimal number corresponding tothe actual address.

After the latch stop has been set cutting off the motor 12 and the ringout detection circuitry 45 transmits a signal along line 45B indicatingthat the stage has settled out, means are provided for energizing thetool 10 to effect a deflection of the tool an amount equal to theremaining difference between the desired position and the actualposition of the workpiece so as to enable precise positioning of thetool relative to the workpiece. To this end, and referring first to FIG.28, a gate (fine mode) 76, which is part of the tool control circuit 75(see FIG. 1) is enabled allowing at least the low order bits indicativeof the difference between the actual and the desired position, to beapplied to a digital converter 80, which converts, in the presentinstance, the digital information and applies the same to a tool driver90, for efiecting motion of the tool 10. Turning now to FIG. 13, thegate 76 includes a pair of AND gates 77 and 78 and a gating bus 79, theAND gate 77 receiving a first enabling input along line 54A from theoutput 54 of the auto/minual switching gate 50 (see FIGS. 2A, 2B and1]). The second input to the AND gate is 35Bl from the latch stop 35,the signal from the latch stop being up when the condition is determinedthat the difference between the desired and the actual address of thestage or table is within the site. The third input to AND gate 77 isfrom the ring out detection circuit 45, notably input 4582, it beingrequired as is conventional with AND gating circuitry, that all inputsbe up before an output 77A is transmitted therefrom. The output 77A ofAND gate 77 applies a first input 77Al to AND gate 78. The second inputto AND gate 78 is transmitted from a line operation complete circuit 97,which includes an AND gate 98 and an inverter 99. The only time that theinverter 99 receives an up signal on its input 99A so as to apply a downsignal at its output 99B is when the input is up and that occurs onlywhen the actual address B equals the command address A. (A' A B orDesired Position minus Actual Position). Thus when there is a differencebetween the actual and desired address the output from along line 998 ofthe inverter 99 is an up signal which, along with the up signal on input77A1 of the AND gate 78 enables the AND gate permitting it to transmit asignal 78A along line 78A to enable a gating bus 79. The gating busdescribed previously in reference to FIG. 11 allows transmission of datawhen the enable line, in this case 78A, is up. The gating bus 79receives the output signal (A') from the digital error generator 31,which input is designated 31C for purposes of identification. Thedifference signal A enters into digital converter 80 (FIG. 13) via bus8I, the least significant digit which is comprised, as is conventional,of four inputs lines representative of the bits 2 2 and represented by abox labelled LSD, the next most significant digit, i.e., the leastsignificant digit LSD-H also representing four lines againrepresentative of the bits 22"*, and the least significant digit LSD 2which is also comprised of four wires 2"2 bits. The input as illustratedis inherently a multiple, the magnitude of the least significant bitsbeing times 1, for the next most significant bits of course times 10,and for the most significant bits times I00, the output being in digitalform (BC'K). The input A from bus 81 enters into an adder or BCD tobinary converter 82 which places the output in a binary form to afunction multiplier 83 which multiplies the binary output of the adderto obtain the proper number of steps of, for example, a stepping motorper digit. [It should be noted that the adders and function multipliersare standard off the shelf items and comprise conventional adder andfunction multiplier circuitry,] For example the adder 82 may be aDigital Equipment Corporation (DEC), Maynard, Mass. M230 as shown inDECs Logic Handbook (1972) at pages 62 and 63. The function multipliermay be, for example, a Texas Instrument 4 bit by 4 bit parallel binarymultiplier such as illustrated on pages 496 and 497 of Texas Instruments"ITL Data Book" for Design Engineers, I973.

The output of the function multiplier 83 is applied to driver circuitry(see FIG. 2B, and FIG. 13) to apply the correct number of steps to thetool 10 to bring the tool into the proper position relative to theworkpiece. To this end and referring once again to FIG. 13, the driver90 comprises a comparator 91, to which is applied the output of thefunction multiplier 83 along output bus 83A, and to which is applied theoutput from a pulse counter 92. A square wave oscillator, sometimesreferred to as pulser or clock 93 applies a pulse to the pulse counter92 as through output 938, the output 92A of the pulse counter 92 beingapplied to the comparator 91. The pulse counter either adds orsubstracts each pulse from its prior total depending upon whether thedifference address A is greater than or less than the actual address B,as indicated by inputs 91A or 918. (For information purposes, the lineshave been designated A greater than B and A less than B). The

outputs of the comparator along lines 91A and 91B is also applied aswell :rs a driver 94 to control the direction of the driving means whichprovides the voltage and current necessary for driving, for example, thestepping motor clockwise or counterclockwise associated with the tool10, in the illustrated instance stepping motor (see FIGS. 13 and 4).

The comparator has a tertiary output along line 91C which is applied tothe fine mode complete circuitry 97, and more specifically to theinverter 99, the output 91C branching into an input 99A to the inverterand to a second branch line 99B which is applied, as noted, to theautomatic/manual switching gate 50. As heretofore described, when theactual address equals the desired address the output of the comparatoralong line 91C goes up causing the inverter to go down providing aninput to AND gate 98 and to 78 disabling those gates. For ease ofconstruction, the modules in the driver may be off-the-shelf items suchas:

Comparator 91, DEC M168, pp. 48 and 49 of the DEC Logic Handbook (Supra)Pulse Counter 92, DEC M236, pp. 66, 67 of DEC Logic Handbook (Supra)Pulser 93, DEC M401, pp. 80, 81 of DEC Logic Handbook (Supra) Driver 94,DEC K202, pp. 166, 167 of DEC Logic Handbook (Supra) The control unit100 (FIG. I) may include an automatic and manual control to effect thenecessary or desired movement of the stage 11 and then the tool asheretofore described. The automatic control may include, for example, apunched or magnetic tape input as well as card either magnetic orpunched which automatically inputs the automatic manual switching gate50 with the required digital information of the desired address. In thesame manner, the manual input may include a conventional typewriter likeinput to input the system with digitized information as to the desiredposition of the stage 1], or thumb wheel switches. A typical example ofa tape drive that may be employed is an IBM 5028 operator station, or anIBM 2501 card reader while a typical example of the manual input systemis a Cherry Electrical Products Corp. Series L-2O lever wheel switch.

It should be recognized that other control systems may be employed. Forexample, and as illustrated in FIG. 15, a process control computer 120,such as the IBM System 7, may be employed as a basic input to theautomatic manual switching gate, the computer having a first output bus121 for inputting the desired address to the switching gate 50, a firstinput bus 122 from the switching gate 50 to indicate, if desired, theactual address of the stage, and a control and status bus 123, whichmay, for example, input the switching gate 50 with information such assetting the gain control through line 29A (see FIGS. 28 and 2C) giving adirect input to gate 76 as through bus 54A (see FIG. 28) etc. It mayalso receive status information from the various parts of the System,for example, polarity reversal conditions as when the stage hits a limitstop, notification from the fine operation complete circuit, etc.

The process control computer 120 may be controlled by a data processingcomputer 124 (such as an IBM System 370, Model 145). The data processingcomputer may be also utilized, in this connection, to provide an output125 to a conventional digital interface 126 for applying work or controlinformation along output bus 127 to, for example, the digital converter80. In this manner the tool, after reaching its desired position, may becontrolled as to its operation on the workpiece W held on the stage 1].To this end, an indication may be taken from the fine operation completecircuit 97 that fine positioning of the tool is complete, such asindication being used to trigger the commencement of actual work on theworkpiece W by the tool. In this connection, it should be noted that themanual address and control entry panel 128, which operates in the samemanner as the manual control described heretofore, acts as a bypasssystem for controlling the position of the tool and permits, entrydirectly into the switching gate 50 as through bus 128A, as well as control information through input line 128B. Manual control of the loop maybe also utilized to apply an input to the manual address and controlentry panel from the manual keyboard 129, in the illustrated embodimentthe keyboard 129 receiving its actual address information as a feedbackloop from the display of the desired address 57 as through line 57A. Thekeyboard may also be used to control the tool by applying a second input129 A from the keyboard to the digital interface 126.

It should be recognized that the tool and control circuit therefore maytake many forms, for example, in the preferred embodiment the digitalconverter may take the form of a digital to analog converter 80A (seeFIG. 14) which, through an electrostatic deflection amplifier or drive Amay be utilized to control the electrostatic deflection means such asplates 131 and 132 of an electron beam 133 for precise positioning ofthe beam relative to the workpiece W, and also to control the movementof the beam to do work on the workpiece, such as the suitably treatedsilicon wafer. This particular embodiment illustrated in FIG. 14 lendsitself to the use of the embodiment illustrated in FIG. 15 wherein thedata processing computer, after initial positioning of the E-beamrelative to the workpiece in a precise location, is capable of writingcircuitry onto suitably covered chips in a silicon semiconductor waferin a predetermined manner. In the embodiment illustrated in FIG. 14, thefine operation complete circuit 97A may be substantially the same asthat disclosed in FIG. 13, the electrostatic deflection amp 90A beingnothing more than an operational amplifier, the fine operation completecircuit being actuated when the difference between actual and desiredaddress is equal to zero, or some ver small value.

Thus the apparatus of the present invention provides a means ofaccurately positioning a tool relative to a workpiece in a minimum oftime so as to enable maximum thru put of operations of the tool relativeto the workpiece.

Although the invention has been described with a certain degree ofparticularly, it is understood that the present disclosure has been madeonly by way of example and that numerous changes in the details ofconstruction, the combination and arrangement of parts, and the methodof operation may be made without departing from the spirit and scope ofthe invention as hereinafter claimed.

What is claimed is:

1. Apparatus for positioning first and second elements in a precisepredetermined relationship one to the other, said apparatus comprising:positioning means for positioning said first element into apredetermined site, a control means having a signal output indi

1. Apparatus for positioning first and second elements in a precisepredetermined relationship one to the other, said apparatus comprising:positioning means for positioning said first element into apredetermined site, a control means having a signal output indicatingthe desired position of said first element relative to said secondelement from a fixed reference; position indicating means to provide asignal indication of the actual position of said element at any one timerelative to said reference; comparing means having an output signalresponsive to the difference between the signal output of said controlmeans and the signal output of said position indicating means; andposition drive means to drive said positioning means in response to saiddifference signal until said first element is within said predeterminedsite; and means for stopping said positioning means upon said differencesignal reaching a predetermined value such as to indicate that saidfirst element is within said site; second positioning means responsiveto said difference signal for positioning said second element in aprecise location relative to said first element.
 2. Apparatus inaccordance with claim 1 wherein said means for stopping said positioningmeans includes latch stop means, said latch stop means operative, uponsaid difference signal reaching said predetermined value, to preventfurther driving of said position drive means until said latchedcondition is removed.
 3. Apparatus in accordance with claim 2 includinga position stop generator, means for applying a signal from saidcomparing means to said stop generator, said signal indicative of thedifference between the actual position of said first elemEnt and itsdesired position, and means for applying a latch signal from saidposition stop generator to said latch stop means upon said differencesignal reaching a predetermined value.
 4. Apparatus in accordance withclaim 1 including gate means, and wherein said means for stopping saidpositioning means includes latch stop means; said latch stop meansoperative, upon said difference signal reaching said predetermined valueto latch said drive means to inhibit further driving of said positiondrive means until said latched condition is removed; and a second inputto said gate means from said latch stop means when said latchedcondition occurs.
 5. Apparatus in accordance with claim 4 includingmeans for indicating when said second element arrives at said preciselocation relative to said first element, and means for applying saidindication to said gate means to thereby open said gate.
 6. Apparatus inaccordance with claim 2, including stop switch means to interrupt saiddifference signal to said position drive means in response to said latchstop means.
 7. Apparatus in accordance with claim 6 including stop logicmeans coupled to said stop switch means; said stop logic meansresponsive to any one of a plurality of inputs to actuate said stopswitch means to interrupt said difference signal to said position drivemeans, one of said inputs being coupled to said latch stop means. 8.Apparatus in accordance with claim 7 including limit means associatedwith the position of said first element; means connected to said limitmeans for applying a second input to said stop logic when said firstelement has been moved a predetermined distance.
 9. Apparatus inaccordance with claim 1 wherein said first element positioning meanscomprises a motor, said position drive means comprising a motor driveamplifier and means for applying velocity feedback to said amplifier tolimit the speed of said motor.
 10. Apparatus in accordance with claim 3including gain control means to limit the velocity of said motor. 11.Apparatus in accordance with claim 1 including means connecting saidcontrol means to said second positioning means, means for indicatingwhen said second element arrives at said precise location, whereby saidcontrol means may directly control the movement of said second element.12. Apparatus in accordance with claim 1 wherein said comparing meanscomprises a digital error generator, and said signal output of saidcontrol means for indicating the desired position of said first elementand the actual position of said first element of said position indicatormeans is expressed in binary coded decimal numbers.
 13. Apparatus inaccordance with claim 12 including a digital to analog converter meansintermediate said digital error generator and said position drive means.14. Apparatus for positioning a workpiece and tool in a precise locationrelative to each other by positioning in a predetermined site saidworkpiece and then positioning the tool in a precise predeterminedposition relative to said site, said apparatus comprising: at least oneclosed loop servo system connected to said workpiece, positioning meansfor positioning said workpiece in a predetermined site, positionindicating means for determining the actual position of said workpiecerelative to a fixed reference, and a signal output from said positionindicator means indicative of said actual position of said workpiece atany one time; and error generator, and means for providing to said errorgenerator the desired position of said workpiece relative to saidreference; and means for inputting said error generator with said signaloutput from said position indicator means, and means in said errorgenerator to produce a difference signal therefrom indicative of thedifference between the actual position of said workpiece and the desiredposition of said workpiece; and means responsive to said differencesignal to actuate said positioning means until said difference signal issuch that said workpiece is in said predetermined site; and first meansresponsive to said difference signal, when said workpiece is in saidpredetermined site to stop said positioning means, and second meansresponsive to said difference signal, when said positioning means hasstopped, to effect movement of said tool until said difference reaches apredetermined second lower value to thereby effect precise positioningof said tool relative to said workpiece.
 15. Apparatus in accordancewith claim 14 including latch stop means for latching said positioningmeans in said stopped position.
 16. Apparatus in accordance with claim15 including stop switch means to interrupt said difference signal tosaid positioning means in response to said latch stop means. 17.Apparatus in accordance with claim 16 including stop logic means coupledto said stop switch means; said stop logic means responsive to any oneof a plurality of inputs to actuate said stop switch means to interruptsaid difference signal to said positioning means, one of said inputsbeing coupled to said latch stop means.
 18. Apparatus in accordance withclaim 17 including first and second limit means associated with theposition of said workpiece; means connected to said limit means forapplying a second and a third input to said stop logic when saidworkpiece has moved a predetermined distance.
 19. Apparatus inaccordance with claim 18 including presettable gain control means tolimit the speed of said positioning means.
 20. Apparatus in accordancewith claim 16 wherein said workpiece is mounted on a stage, and saidpositioning means includes a motor for moving said stage in apredetermined line of movement.
 21. Apparatus in accordance with claim20 wherein said workpiece comprises a semiconductor wafer, and said toolcomprises an electron beam means.